R ELATED L ITERATURE

1.2.1 Comb Drive with Silicon-Based

There were many literatures on comb drive. In 1989, comb-drive were reported by Tang et al [1]. The design of comb drive actuators is shown in Figure 1-1. The concept of actuation was using the relation between electrostatic force and spring force. Voltage was the input signal. Different voltage applied on the different combs and resulted in electrostatic force between active and immobile combs. After the balance between electrostatic force and spring force provided form the eight beams suspend on the two side of the active combs part, the output signal displacement was obtained. By matching the frequency of the input signal with the resonant frequency of comb drive structure, large displacement was obtained.

In order to improve the performance of comb drive actuators, many researches were published to show the ways to achieve it. Using comb drive as a resonator, the resonant frequency is the most important point needs to be improved. From the frequency equation, the frequency depends on the spring constant and the mass of the system. The spring constant and the mass of the system can be adjusted by the dimensions and the material of the structure. Actually, searching for the suitable materials is the main destination.

In Tang’s fabrication process, low pressure chemical vapor deposition (LPCVD) polycrystalline silicon was utilized as the structural material [1]. The operation temperature

of LPCVD polycrystalline silicon was above 600 . The high temperature process induced ℃ thermal stress and low deposition rate were the issues of polycrystalline silicon. Biebl et al presented a method to improve the deposition rate, resistivity, and residual stress [2]. By controlling the ratio of phosphine to silane and rapid thermal annealing (RTA) process, the 42 Å/min deposition and 0.8mΩcm resistivity were obtained. A cantilever beam with 2 µm in thickness and 1000µm in length showed an end deflection of less than 0.2µm.

Franke et al used polycrystalline germanium instead of polycrystalline silicon as structural material in 1999 [3]. The fabrication process included LPCVD and RTA. By this process, low resistivity and tensile polycrystalline germanium film was obtained. The testing results showed that RTA process shifted the film stress form tensile to compressive.

So by well control of deposition and annealing condition, the film stress could be minimized.

The polycrystalline silicides were bought up as the structural materials. Tri-layer polycrystalline SiGe structure could reduce the strain gradient [4]. Polycrystalline SiC provided high Young’s modulus up to 710 GPa [5]. These two materials both deposited by LPCVD and needed RTA process to reduce thermal stress.

Comb drives were made by surface micromachining in these literatures above. There were some comb drives made by bulk micromachining. The most popular example was using the SOI wafer to achieve it [6][7][8]. The thick silicon layer was utilized as the structure and fabricated by bulk micromachining, so the high aspect ratio combs were obtained. This method was usually used as the vertical driving comb drive because it provided larger area for comb sidewall.

Figure 1-1 comb structures with spring beams on each side[1].

1.2.2 Nickel and Nickel Nanocomposite

Being a new technology, electroplating nickel provides many advantages. High deposition rate, easy stress control, low deposition temperature, and low resistivity are the advantages of electroplating nickel instead of polycrystalline silicon [9][10]. By choosing the suitable fabrication process, it is well suited for post processing on preprocessed CMOS wafers.

In the development of nano particles in metal, based on electroplating nickel, there were some materials being added into the solution of electroplating. In 2002, Teh et al used the ceramic particles as the additive to electroplate the nickel ceramic composite film [11].

Ceramic particles, either in the form of cordierite, or iron oxide, dispersed well within the solution. After fabrication of the nickel ceramic film, the testing result showed that composite film significantly reduced the mismatch of thermal expansion between nickel and silicon. Fortunately, the Young’s modulus, Berkovich hardness, and electrical resistivity were maintained.

After nickel ceramic composite film, Teh et al used diamond and cordierite as the additive [12]. In Figure 1-2, the left microresonator was made by nickel and the right one was made by nickel cordierite. Obviously, the nickel cordierite microresonator was residual stress free because of the better thermal compatibility with silicon. By adding various concentration of diamond particles, it was found that higher diamond concentration obtained the film with more compressively stress.

In 2003, Tsai et al followed Teh’s research and used nickel and diamond nanoparticles as the materials of electro-thermal microactuators [13]. The E/ρratio of microactuators could be enhanced 7.1 times with diamond concentration of 2 g/l. Comparing to device made of electroplated nickel, the microactuators with diamond concentration of 2 g/l could reduce 73% power requirement for 3µm displacement of cantilever beam.

The similar research is taken by Shen in 2004 [13]. The Ni-P-CNT and Ni-P-Diamond films are gotten by electroless. The E/ρ ratios reach 3.9 and 3 times for Ni-P-CNT and Ni-P-Diamond individually. The electrical conductivities are 1.903x10-6Ω-m and 1.399x10-6Ω-m.

Comparing metal-based with surface silicon-based micromachining, the deposition rate and thermal stress will be improved. Although using bulk silicon-based micromachining is good for the aspect ratio and the fabrication, it is not easy to integrate comb drive with IC

process and the cost SOI wafer is high. Using metal based process will solve these problems in bulk silicon-based micromachining.

Figure 1-2 (a, left) a partially released nickel microresonator. (b, right) a full released nickel cordierite microresonator[12].